Okay, I ran some tests the other night on M81-M82 using the following recommended exposure settings:
Median ADU shown in SGP -
Gain 0 Offset 10: 400 ADU
Gain 75 Offset 12: 550 ADU
Gain 139 Offset 21: 850 ADU
Gain 200 Offset 50: 1690 ADU
Gain 300 Offset 50 : 2650 ADU
Better to be a little high than low
For other Gains or offsets use the formula Jon indicated: MinDN16 = (((ReadNoise * 20) / Gain) + BiasOffset) * 2^16/2^Bits
Just pick "X" for dithering every "X" frames to be 5% of the number of subs you are going to collect. If you are collecting 20 subs, you should dither every frame. If you are collecting 50 subs, dither every 2 frames (OK the rule should say "round down"), etc.
In SGP you want your flats to measure 12k-18k DN.
Use Dark Flats instead of Bias if Flats long.
Get new calibration frames when changing driver.
The attached image is a cropped calibrated (dark, bias and flat at 15k) 1 minute sub in a fairly light polluted sky. I use the IDAS D2 LPS. The image was taken at a Gain of 0 and Offset of 10. The Fit image stats were as follows:
The center of M81 reads as follows:
The bright center of M82 reads as follows:
Following the advice of that thread, the targets M81 and M82 seem woefully underexposed. A stack of ~150 subs did not yield a useful picture as I had to stretch and stretch the image to bring out the galaxy arms and the BG by that point looked horrible.
I think I understand the proposed concept of keeping the BG at about 20x the read noise or higher so as to not clip too much data or reducing the dynamic range. But it seems that this theory breaks down in actual practice. It seems to me that clipping star data cannot be avoided if much of the target details are going to be brought out. If I increased the exposure time perhaps by 15-30 seconds, the star data starts getting clipped. I am not sure it is a bad thing to clip star data if you are not going to be able to reasonably bring out target details. What is the harm of clipping star data? I sure it is possible that I am doing something wrong here. If the thread exposure time is correct, It would seem to me that I would need upwards of 500 or more subs to get the target detail out and I tend to doubt that would help if a stack of 150 was unusable.
It also seems to me that the target stats are way too low to bring out much information even though the calibrated BG is a little higher than the thread recommends. Again, I may be doing something wrong or not understand this exposure thing. I am wondering if a 3 minute sub would yield much better results even though the BG stats would be in the 3-5k at this gain setting (0 and 10).
I am probably opening up a can of worms here, but I am trying to learn this camera and my new EON 115 scope.
If you guys have recommendations as to what exposure to use without blowing the BG out, that would be helpful as well.
I know Jon Rista can probably tell me where I am going wrong as I know he likes the short exposure times.
Edited by Kaos, 21 May 2017 - 09:30 PM.